Re: [PATCH v1 2/2] swiotlb: Fix slot alignment checks

[Kelsey Steele]

On Thu, Apr 06, 2023 at 04:42:00PM +0200, Petr Tesarik wrote:
> Hi Kelsey,
> 
> On 4/6/2023 6:52 AM, Kelsey Steele wrote:
> > On Wed, Apr 05, 2023 at 07:50:34AM +0200, Petr Tesa????k wrote:
> >> On Wed, 5 Apr 2023 07:32:06 +0200
> >> Petr Tesa????k <petr@xxxxxxxxxxx> wrote:
> >>
> >>> On Wed, 5 Apr 2023 05:11:42 +0000
> >>> Dexuan Cui <decui@xxxxxxxxxxxxx> wrote:
> >>>
> >>>>> From: Petr Tesa????k <petr@xxxxxxxxxxx>
> >>>>> Sent: Tuesday, April 4, 2023 9:40 PM    
> >>>>>>> ...
> >>>>>>> Hi Petr, this patch has gone into the mainline:
> >>>>>>> 0eee5ae10256 ("swiotlb: fix slot alignment checks")
> >>>>>>>
> >>>>>>> Somehow it breaks Linux VMs on Hyper-V: a regular VM with
> >>>>>>> swiotlb=force or a confidential VM (which uses swiotlb) fails to boot.
> >>>>>>> If I revert this patch, everything works fine.  
> >>>>>>
> >>>>>> The log is pasted below. Looks like the SCSI driver hv_storvsc fails to
> >>>>>> detect the disk capacity:    
> >>>>>
> >>>>> The first thing I can imagine is that there are in fact no (free) slots
> >>>>> in the SWIOTLB which match the alignment constraints, so the map
> >>>>> operation fails. However, this would result in a "swiotlb buffer is
> >>>>> full" message in the log, and I can see no such message in the log
> >>>>> excerpt you have posted.
> >>>>>
> >>>>> Please, can you check if there are any "swiotlb" messages preceding the
> >>>>> first error message?
> >>>>>
> >>>>> Petr T    
> >>>>
> >>>> There is no "swiotlb buffer is full" error.
> >>>>
> >>>> The hv_storvsc driver (drivers/scsi/storvsc_drv.c) calls scsi_dma_map(),
> >>>> which doesn't return -ENOMEM when the failure happens.  
> >>>
> >>> I see...
> >>>
> >>> Argh, you're right. This is a braino. The alignment mask is in fact an
> >>> INVERTED mask, i.e. it masks off bits that are not relevant for the
> >>> alignment. The more strict alignment needed the more bits must be set,
> >>> so the individual alignment constraints must be combined with an OR
> >>> instead of an AND.
> >>>
> >>> Can you apply the following change and check if it fixes the issue?
> >>
> >> Actually, this will not work either. The mask is used to mask off both
> >> high address bits and low address bits (below swiotlb slot granularity).
> >>
> >> What should help is this:
> >>
> > 
> > Hi Petr, 
> > 
> > The suggested fix on this patch boots for me and initially looks ok,
> > though when I start to use git commands I get flooded with "swiotlb
> > buffer is full" messages and my session becomes unusable. This is on WSL
> > which uses Hyper-V.
> 
> Roberto noticed that my initial quick fix left iotlb_align_mask
> uninitialized. As a result, high address bits are set randomly, and if
> they do not match actual swiotlb addresses, allocations may fail with
> "swiotlb buffer is full". I fixed it in the patch that I have just posted.
> 
> HTH
> Petr T

I pulled the patches from dma-mapping after your fix got applied and
everything appears ok and goes back to the way it was; so no other
errors to report. :) Unfortunately still getting the "swiotlb buffer is
full" messages during kernel builds, though that was happening before
your patches hit.

Thanks so much, Petr!

Cheers, 
Kelsey.